1. Field of Invention
The present invention relates to a power detecting device, a power supply device and a reference voltage generator, and more particular, to a device that controls power-on resetting by detecting a power voltage.
2. Description of the Related Art
Recently, due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. In the power supply system of the LCD, a power-on reset circuit is usually utilized for providing a reset signal to reset the LCD when power is started, and ensuring the LCD operates under a known state.
The power-on reset circuit is usually implemented by using a resistor-capacitor (RC) circuit and a Schmitt trigger circuit. As the increase of the power, the RC circuit generates a charging signal to the Schmitt trigger circuit. The Schmitt trigger circuit generates a rising edge of the reset signal when the charging signal reaches a first threshold value, and then generates a falling edge of the reset signal when the charging signal reaches a second threshold value to reset the device. A time that the charging signal changes from the first threshold value to the second threshold (i.e. a pulse width of the reset signal) should be long enough to complete the reset operation of the internal circuits of the device.
However, the charging signal is a ramp signal, which a rising rate of the charging signal is determined according to the resistance and the capacitance of RC circuit. If the power increases rapidly, the resistance and the capacitance of RC circuit should be increased for obtaining the reset signal which its pulse width is long enough. Nevertheless, if the power increases slowly, the charging signal may not drive the Schmitt trigger circuit to operate, so that the reset signal can not be generated.
In another aspect, the power-on reset circuit can use a comparator to compare a divided voltage, obtained from dividing the power through series-connected resistors, with a reference voltage generated by a bandgap voltage generating circuit, and thereby generates the reset signal to reset the device. That is to say the comparator generates the reset signal to reset the device when the divided voltage reaches the reference voltage. However, the operation of the bandgap voltage generating circuit is also driven by the power. In the beginning of the power on the rise, or when the power is transiently supplied to an over load, the bandgap voltage generating circuit is likely affected by the unstable power to generate a smaller reference voltage, and thus the power-on reset circuit may be started incorrectly.
When the LCD is reset, pixel capacitors on a display panel are usually discharged for displaying a black frame. The incorrect operation of the power-on reset circuit causes residual voltages stored within the pixel capacitor since there's not enough time for the pixel capacitors to discharge. As a result, deficient image may occur on the displayed frame. The designers should make efforts on the improvement of detecting the power.